I know you're wondering WHAT is field herping? That is a term used by people who enjoy going out into nature, and looking for wild snakes/reptiles and amphibians in their natural environment. The point is not to catch or hunt them, in fact "herpers" (short for herpetoculturists) follow the mantra "leave only footprints and take only photographs." You do not want to disturb their habitat or the snakes themselves, and in addition, if they happen to be protected/endangered you could face serious fines.
Did you know that when BART (Bay Area Rapid Transit) was building the SFO station, they accidentally killed an endangered San Francisco Garter Snake? That little mistake cost them over $1M in fines, and halted construction while they combed the area to make sure no more were present. So regardless of the fact that they are completely harmless, do not disturb these beautiful creatures; just take a photograph from a safe distance, as this herper did when they spotted one:
Before you head out into the wild looking for reptiles, be sure to pick up a book on identifying the local species. It is very important to know what you're finding, especially if your region has any venomous native species. Other than that, all you need to get started is some nice weather, a good camera, and a hiking buddy!